Modified bubbling transfer method for graphene delamination

ABSTRACT

In the BB transfer, or so called electrochemical delamination process, a transfer film is firstly spray-coated on a stack formed by two graphene sandwiching a metal (Cu or Cr) foil as a protection layer. Then, direct current (dc) voltage is applied to the first stack as a cathode and an anode (from be a platinum wire, a carbon rod, or others) in an electrolyte aqueous solution. With application of the electrolysis potential, hydrogen bubbles appear at the graphene/metal foil interfaces, while oxygen bubble appear at the anode due to the reduction of water. These H 2  bubbles provide a gentle but persistent force to detach the graphene film from the copper foil at its edges, and the process is aided by the permeation of the electrolyte solution into the interlayers as the edges delaminate.

FIELD OF THE INVENTION

The present invention relates to graphene delamination, and in particular to a modified bubbling transfer method for graphene delamination.

BACKGROUND OF THE INVENTION

Chemical vapor deposition (CVD) method for graphene growing provides a way for graphene industrial applications because it produces large area, flexibility, high transparency, and high electrical conductivity graphene film (referring to Li XS et al. Science; US patent: US20110091647). However, the traditional transfer method of delaminating graphene from a metal substrates usually includes using polymethylmacralate (PMMA) as a carrier material and a chemical etching step to remove the metal substrate (Li XS, et al. Nano Letters) . The PMMA film as carrier material is fragile and expensive. It is replaced with other transfer used polymer film, which could be selected from a group of materials including nitrocellulose lacquer, polyurethane, epoxy, and other ketone removable polymer coatings (referring to U.S. patent application Ser. No. 13/603,786 to Stehle et al., entitled “Methods for Transferring Graphene Films and the Like between Substrates). The etching step not only increases the production cost but also causes a lot of waste and extends the transfer time consumed. An electrochemical method to delaminate graphene film from a metal substrate, such as Cu foil, is developed. This bubbling (BB) transfer, or so called electrochemical delamination method uses hydrogen bubble generated during water electrolysis process on the Gr/Cu interface to force the detachment of graphene film from the Cu foil.

Advantages of this technique are industrial scalability of the process and reusability of the Cu foil in multiple growth and delamination cycles. Unlike the tradition transfer method, BB transfer does not consume copper foil, copper etchant, or other chemical so it makes the process fast and clean. The process also serve as a self-improving electrochemical polishing and thermal restructuring of the copper foil so as to induce multiple reuses of copper foils.

However, the prior art method has some disadvantages: (1) Delaminating speed greatly decreases with increment of delamination areas. (2) Control of the generating speed and the break force for each bubble is not easy, but they are very important for final graphene quality. (3) Bubbled still generate on a top of the graphene layer after it separates from Cu, while it still stay in the electrolyte so as to cause graphene loss and film break.

SUMMARY OF THE INVENTION

The present invention uses transfer cloth to drive the cathode and anode as close as they could be without causing short circuit, so as to increase delamination speed and reduce non-uniformity caused by the bubbling process. Advantages of the modified bubbling transfer method are that: (1) reaction speed is fastened, in that the delamination speed could be controlled to 12″/min; (2) reaction efficiency is high, in that, the graphene layer will be transferred to the transfer film and covered by a protective cloth for package; (3) The quality is preferred than prior art way, in that: transferred graphene film keep its flexibility, high transparency, and high electrical conductivity as tradition method transferred graphene.

To achieve above object, the present invention provides a modified bubbling transfer method for graphene delamination comprising the steps of: preparing the electrolyte solution for water electrolysis, pre-coating a transfer film on the upper graphene film of a first stack of an upper graphene film, a metal foil and a lower graphene film and connect with conducting wire to serve as a positive electrode, preparing a negative electrode; locating the positive and negative electrodes in the electrolyte solution and connecting to a DC power supply; locating a non conductive transfer cloth between the two electrodes for separating the two electrodes; switching on the DC power supply to begin the electrochemical delamination process, a second stack formed by the transfer film and the upper graphene film gradually separating from the metal foil; lifting the second stack of the transfer film and the upper graphene film from the metal foil with the transfer cloth and the negative electrode being parallel as close as they could be, and the transfer cloth gradually attaching to a bottom of the first graphene film which is separated from the metal foil and an exposed upper surface of the metal foil; dipping the lift up the third stack formed by the transfer film, the first graphene film and the transfer cloth into de-ionized water to remove electrolyte residue on the surfaces of the third stack, and drying the third stack to remove de-ionized water thereon for final package.

A voltage between the two electrodes being controlled within 20V with current density within 1A/square inch. The metal foil is a copper foil or a Cr foil. The negative electrode is a platinum wire or a carbon rod.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the step of coating transfer material in the method of the present invention.

FIG. 2 shows the step of beginning electrochemical delamination process according to the present invention.

FIG. 3 shows the middle process in electrochemical delamination process according the present invention.

FIG. 4 shows the end state in electrochemical delamination process of the present invention.

FIG. 5 shows the cleaning step of the present invention.

FIG. 6 shows the step of drying according to the present invention.

FIG. 7 shows the step of get final product to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order that those skilled in the art can further understand the present invention, a description will be described in the following in details. However, these descriptions and the appended drawings are only used to cause those skilled in the art to understand the objects, features, and characteristics of the present invention, but not to be used to confine the scope and spirit of the present invention defined in the appended claims.

In electrochemical delamination process, a transfer film is firstly spray-coated on a stack formed by two graphene sandwiching a metal (Cu or Cr) foil as a protection layer. Then, direct current (dc) voltage is applied to the first stack as a cathode and an anode (from be a platinum wire, a carbon rod, or others) in an electrolyte aqueous solution. With application of the electrolysis potential, hydrogen bubbles appear at the graphene/metal foil interfaces, while oxygen bubble appear at the anode due to the reduction of water. These H2 bubbles provide a gentle but persistent force to detach the graphene film from the copper foil at its edges, and the process is aided by the permeation of the electrolyte solution into the interlayers as the edges delaminate.

Reaction in the positive electrode is:

2H2O(1)+2 e=H2(g)+2OH−(aq);

Reaction in the negative electrode is:

4OH−(aq)−e=2H2O(1)+O2(g)

The present invention directs to a more effective, lower cost, clean, environmental friendly, large scalable method to delaminate the graphene film from metal substrate.

Referring to FIGS. 1 to 6, the process of the present invention comprises of the following steps of

Preparing the electrolyte solution 150 for water electrolysis, pre-coating transfer film 130 on a first stack of an upper Graphene film 120/a Copper foil 110/a lower Graphene stack 120 and connecting the first stack with conducting wire (not shown) to serve as a positive electrode, wherein the copper foil may be replaced by a Chromium (Cr) foil, as illustrated in FIG. 1;

Using a platinum wire or carbon rod as a negative electrode 160; Putting both electrodes in the electrolyte solution 150 and connect to a DC power supply (not shown);

Arranging a non conductive transfer cloth 170 between the positive electrode and the negative electrode for separating the two electrodes, referring to FIG. 2;

Switching on the power supply to begin the electrochemical delamination process, the voltage between the two electrodes is controlled within 20V with current density within 1A/square inch; wherein in the electrochemical delamination process, with application of the electrolysis potential, hydrogen bubbles appear at an interface of the graphene film 120 /Copper foil 110, while oxygen bubble appear at the anode 160 (negative electrode) due to the reduction of water; these hydrogen bubbles provide a gentle but persistent force to detach the upper graphene film 120 from the copper foil 110 at its edges, a second stack containing the transfer film 130 and the upper graphene film 120 will gradually separate from the copper foil 110 (referring to FIG. 3);

Referring to FIG. 4, lifting the detached second stack from the copper foil 110 with the transfer cloth 170 and the negative electrode 160 being parallel as close as they could be, and the transfer cloth 170 gradually attaches to a bottom of the upper graphene film 120 separated from the copper foil 110 and an exposed upper surface of the copper foil 110;

Dipping a third stack of the transfer film 130, the upper graphene film 120 and the transfer cloth 170 into de-ionized water 180 to remove electrolyte residue on surfaces of the third stack (see FIG. 5), and then Drying the third stack to remove de-ionized water thereon for final package, see FIG. 6.

In above mentioned process, the transfer film (AR) 130 could be selected from a group containing nitrocellulose lacquer, polyurethane, PMMA, epoxy, and other ketone removable polymer coatings. The electrolyte could be 0.05 mM to 0.1 M water solution of K2SO4, K2S2O8, or NaOH, or other water solvable salt. The electrolysis voltage is controlled within 20 Volts. Preferable, it is adjusted between 4 and 16 Volts. With predetermined proper transfer film 130, electrolyte solvent content, electrolysis current density, the detach speed was adjusted for different graphene size to achieve the best graphene quality.

The distance between the negative electrode and the bubble delaminating Cu (Cr) interface, which serve as positive electrode, decides the delaminating speed. The closer the distance, the faster the delamination could be However, too close distance could cause short circuit. In one aspect of this invention, transfer cloth is applied to separate the positive and negative electrodes, prevents the graphene layer scratch or damage, and also serves as protective layer in the final graphene package and storage.

Proper ultrasonic treatment could be applied the BB transfer set-up for remove the new generated bubble and aid the delamination process without cause graphene loss or film break.

The present invention is thus described; it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A modified bubbling transfer method for graphene delamination comprising the steps of preparing the electrolyte solution for water electrolysis, pre-coating a transfer film on the upper graphene film of a first stack of an upper graphene film, a metal foil and a lower graphene film and connect with conducting wire to serve as a positive electrode, preparing a negative electrode; locating the positive and negative electrodes in the electrolyte solution and connecting to a DC power supply; locating a non conductive transfer cloth between the two electrodes for separating the two electrodes; switching on the DC power supply to begin the electrochemical delamination process, a second stack formed by the transfer film and the upper graphene film gradually separating from the metal foil; lifting the second stack of the transfer film and the upper graphene film from the metal foil with the transfer cloth and the negative electrode being parallel as close as they could be, and the transfer cloth gradually attaching to a bottom of the first graphene film which is separated from the metal foil and an exposed upper surface of the metal foil; dipping the lift up the third stack formed by the transfer film, the first graphene film and the transfer cloth into de-ionized water to remove electrolyte residue on the surfaces of the third stack, and drying the third stack to remove de-ionized water thereon for final package.
 2. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein a voltage between the two electrodes being controlled within 20V with current density within 1A/square inch.
 3. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein the metal foil is a copper foil or a Cr foil.
 4. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein the transfer film is selected from a group containing nitrocellulose lacquer, polyurethane, PMMA, epoxy, and other ketone removable polymer coatings.
 5. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein the electrolyte is a 0.05 mM to 0.1 M water solution of K₂SO₄, K₂S₂O₈, or NaOH, or other water solvable salt.
 6. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein the electrolysis voltage is adjusted between 2 and 16 Volts.
 7. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein with predetermined proper transfer film, electrolyte solvent content, electrolysis current density, the detach speed was adjusted for different size of the graphene films.
 8. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein a distance between the negative electrode and a delaminating Cu (Cr) interface between the metal foil and the second stack as positive electrode, decides the delaminating speed; the closer the distance, the faster the delamination could be
 9. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein the negative electrode is a platinum wire or a carbon rod.
 10. The modified bubbling transfer method for graphene delamination as claimed in claim 1, wherein ultrasonic treatment could be applied the BB transfer set-up for remove the new generated bubble and aid the delamination process without cause graphene loss or film break. 